A vesicle is also referred to as a liposome, and it is a small enclosed compartment formed by an amphiphilic lipid molecule. Thus, it is anticipated that such vesicle will be used for various purposes such as a model cell membrane, a DDS preparation and a microreactor for research usage.
An example of a method for producing a vesicle is a method comprising hydrating a thin lipid membrane formed on the surface of a solid. For example, a thin lipid membrane is formed on a glass substrate, and an aqueous solution is then added thereto, followed by intensive stirring, so as to prepare a vesicle. However, the particle diameters of vesicles obtained by this method are generally non-uniform, and the efficiency of entrapping a desired substance in the vesicle is extremely low. Moreover, it is difficult for this method to prepare a large vesicle having a particle diameter of a micrometer-scale.
On the other hand, as a method that has been frequently used in the production of a large vesicle, an electroformation method disclosed in Non-Patent Document 1 is known. This method comprises applying an electric voltage to a thin lipid membrane formed on a platinum electrode in an aqueous solution to hydrate the thin lipid membrane, so as to produce a vesicle with a diameter between approximately several tens of and several hundreds of μm.
Moreover, as a method for producing a vesicle having a relatively high entrapment yield, a method using emulsion is proposed in Non-Patent Document 2.
Herein, the term “entrapped substance” is used in the present invention to mean a substance entrapped in a vesicle. The term “entrapment yield” is used herein to mean the ratio of substances entrapped in a vesicle to those contained in the finally obtained vesicle suspension as a whole. Furthermore, the term “mean particle diameter” is used herein to mean a particle diameter calculated as a geometric mean of number reference in the number distribution of vesicle particle diameters.
Further, Patent Document 1 proposes a method for producing a microcapsule, which comprises forming a gel layer of an electrolytic complex around a polyelectrolyte solution. In this method, a polyelectrolyte solution (a dispersion phase) in which a given substance is entrapped is disposed on one side of a microchannel, and a solution (a continuous phase) that reacts with the polyelectrolyte solution to form a gel is disposed on the other side thereof. By applying pressure to the dispersion phase, the dispersion phase is supplied as fine particles to the continuous phase, so that a gel layer is formed by a reaction occurring on the surface of the dispersion phase.    Patent Document 1: WO 2004/026457    Non-Patent Document 1: Wick et al.: Chemistry & Biology, Vol. 3, 277-285, 1996    Non-Patent Document 2: Szoka et al.: Proc. Natl. Acad. Sci. USA, Vol. 75, No. 9, 4194-4198, 1978